Chapter 1: Plate Tectonics
Section 1.4: Plates converge or scrape past each other.
I. The Big Idea and Key Concepts
A. Big Idea: The movement of tectonic plates causes geologic changes on Earth.
B. Key Concept 1.1: Earth has several layers.
C. Key Concept 1.2: Continents change position over time.
D. Key Concept 1.3: Plates move apart.
E. Key Concept 1.4: Plates converge or scrape past each other.
II. 1.4: Plates converge or scrape past each other.
A. Tectonic plates push together at convergent boundaries.
1. Here crust is either folded or destroyed
a. When two plates with continental crust collide, they will crumple and fold the rock between them
b. Older, denser oceanic crust will sink beneath another plate
c. The crust melts in the asthenosphere and is destroyed
d. This is the process of subduction
2. Three types of convergent boundaries
a. Where continental plates meet
b. Where two oceanic plates meet
c. Where oceanic and continental plates meet
3. Major geologic events occur at all three types of boundaries
B. Continental-Continental Collision
1. Occurs when two plates carrying continental crust push together
2. Both plates are the same density, so crust folds and crumples along the edges
3. In some cases, the folded crust can be pushed up high enough to form mountains
a. Some of the world’s largest mountains appear along continent-continent boundaries
b. The European Alps began forming nearly 40 million years ago
c. The Himalayas formed when the Indian Plate began colliding with the European Plate
i. Located between India and Tibet
iii. Himalayas and Alps are still forming today; as long as the plates keep moving the mountains will rise
1. Occurs when one plate with oceanic crust sinks (subducts) under another plate with oceanic crust
a. The older plate sinks because it is colder and denser
b. Older crust melts in the asthenosphere under intense heat
c. Deep ocean trenches and island arcs form here
d. Deep ocean trenches
i. Deep canyons that form on the ocean floor as a plate sinks
ii. Most are found in the Pacific Ocean
iii. The Mariana Trench, under the Philippines, is sinking. This trench is the deepest place in the oceans, extending nearly 36000 ft. (6.8 mi.) That is equivalent to running the bases from home plate to home plate (at a major league distance) 100 times!
e. Island Arcs
i. Chains of volcanic islands that form on the top of the plate, parallel to a deep-ocean trench
ii. Include the Philippine Islands, Aleutian Islands, and the Islands of Japan
D. Oceanic-Continental Subduction
1. Occurs when ocean crust sinks under continental crust
a. This crust sinks because it is colder and denser than the continental crust
b. Deep-ocean trenches form here, along with coastal mountains
2. Deep-ocean trenches
a. Youngest trenches are in the Pacific
b. Here, the Pacific Plate is sinking under the North American plate
c. Moving oceanic crust causes underwater earthquakes
3. Coastal mountains
a. As oceanic crust sinks under a continent, the continental crust buckles to form a range of mountains
b. Some are volcanoes, but all parallel the ocean trench
c. The Cascade Mountains of Washington and Oregon
E. Tectonic plates scrape past each other at transform boundaries.
1. These occur mostly on the sea floor near mid-ocean ridges
2. They also occur on land
i. Runs from the Gulf of California through the San Francisco area
ii Marks where the Pacific and parts of the North American plate are moving in opposite directions
F. Why is the theory of plate tectonics so important to geologists?
1. Helps them to explain Earth’s past
2. Helps them to predict future events